And sometimes I do read it to the end (when it's still short) and no one has made the comment I want to make. I post (sometimes editing it a lot) and I find that when I post it, in the interval between when I started composing a reply and when I hit the "post" button, someone else has already gotten in and said it already.
For me it depends on the length of the thread. Sometimes I will read a really long thread and get caught up before posting. If it is a "real time" event (commentary on the Oscars or debates), then such effort is not necessary.
Some lengthy threads get new "recruits" pinged by freepmail to defend a losing position and none of them read the arguments that have already been laid out. It is on these threads that I repeatedly have to offer "proof" of my statements only to have the argument move on to another subject (statement proven, an hour's web search/editing etc, and statement is thusly ignored).